Tellurium rectifier and method of making it



A ril 3, 1956 c. A. ESCOFFERY TELLURIUMRECTIFIER AND METHOD OF MAKING IT Filed Feb. 18, 1952 INVEN TOR. Charles A. Escoffery ATTOI? K United StatesPatent O TELLURIUM RECTIFIER AND lVIETHOD OF MAKING IT Charles A. Escofi'ery, Los Angeles, Calif., assignorto International Rectifier Corporation, Los Angeles, Calif., a corporation of California Application February 18, 1952, Serial No. 272,125 8 Claims. (Cl. 317 -241) Rectifying elements of the dry type are well known.

Such elements have beenrmade, for example, from .semiconducting material such as selenium, copper oxide, magnesium-copper sulfide, silicon and-germanium. I

The selenium, copper oxide and magnesium-copper sulfide elements are dry plate rectifiers classed-as of the area type, as the current flows 'throughplates or layers of substantial area. The silicon and germanium elements are classed as of the point contacttype because the recti fied current flows througharelatively small cross sec tion conductor such as. a cat-whiskerntype of wire, the point of which is pressedagainst the silicon or germanium surface. When substantial'current flow is desired,

the area type is used, asthe pointcontacttype is capable of only very small current flow. r

A common form of an areatypeelement iswthe seleniurn element which ordinarily-comprises la base plate carrying a layer of selenium covered by a counterelectrode. Its rectifying action is generallyattributed to the presence of a barrier or blockinglayer-at thesurface of the selenium contiguous to the counterelectrodez. The element is characterised by a relatively high forward conductivity and a relatively low reverse conductivity.

The several types of rectifiers notedv above have somewhat differing advantages and disadvantages. For example, the selenium rectifier has become'popular for its ability to handle substantial currents at voltages ranging up to around 40 volts R. M. S. per plate, but it is subject to the disadvantage that it is not very suitable for long term operation at ambient temperatures above about 100 C. The other of the above named kinds of rectifierhave been found even less suited for high temperature operation, with thepossible exception of the magnesium-copper sulfide rectifier; The magnesium-copper sulfide rectifier, however, is characterized by a poorer efliciencyand shorter life as compared with the selenium and copper oxide rectifiers.

The growing demand for dry plate rectifiers operable at high ambient temperatures such as 250 C. or higher makes it desirable to provide. a different kind of rectifier than those named above. f h

In accordance with my present invention I have succeeded in producing a dry plate rectifier continuously operable at ambient temperatures as high as about 400 C. and which is capableof substantial rectified current and an efficiency comparable with that of the selenium rectifier. In carrying out my invention I makeuse of tellurium as the active semi-conductor substance. Althrough tellurium has heretofore been suggested in rectifier elements,,it has not heretofore been practical to use it, except to a limited extent as a point contact rectifier.

2,740,925 Patented Apr. 3, 1956 It has not heretofore been successfully used as an area type rectifier capable of substantial current flow. The Hollnagel et al. Patent No. 1,866,351 describes a tellurium rectifier in which tellurium is sandwiched between bimetallic plates. But in it, the tellurium is in the form of thick cast plates which impede the flow of current in the forward direction, and has not resulted in a suc-.-

cessful rectifier. The Ruben patents, 1,751,361 and 1,751,362, show rectifiers' comprising electropositive and electronegative plates wherein the electronegative element can be a telluride. These, however, have never been found practical for use.

I have succeeded in obtaining high efficiency in the use of tellurium by the discovery of methods and arrangements which make it efiicient and practical. In accordance with my invention I apply the-tellurium by'hot-J,

pressing fine powder on a base plate, which may be silvered, and since tellurium does not fiow appreciably I use veryhigh pressures in the pressing which is preferably done at temperatures as high as about 200 C. although temperatures as high as 400 C. or as low as C. or even room temperature might be used. For the 'counterelectrode, an oxide coated magnesium sheet or the like may be used.

The foregoing and other features will be better understood from the following detailed description and the accompanying drawing of which:

Fig. l is a plan view of a base plate used in making a rectifier element according to this invention;

Fig. 2 is a side view in cross section of the plate of Fig. 1;

Fig. 3 is a plan view of the plate covered with tellurium powder;

Fig. 4 shows the element of Fig. 3 in a press; and

Fig. 5 shows the pressed element provided with a counterelectrode. 7

Referring to Figs. 1 and 2 of the drawing, the element is made on a suitable base plate 10 which is ordinarily of sheet metal such as iron, steel, copper or the like, which may be coated with another metal such as silver, copper, or brass 11. Preferably the base plate is not made of aluminum or magnesium unless it is plated or coated with another metal such as silver, copper or brass, over which the tellurium is placed. Although the base plate is shown circuiar in shape it should be understood that it may have some other shape such as square or rectangular. The base plate preferably has its upper surface roughened as by sand blasting to aid in holding the tellurium, and a preferred form is a sandblasted iron plate covered with a coating 11 of silver (Figs. 2 and 3 A layer of fine tellurium powder 12 is spread on top of the coating 11. A mesh powder has been found very satisfactory although coarser grades have also been used, such as 40 mesh.

The tellurium covered plate is then placed on the surface of a heatable platform 13 (Fig. 4) which can conveniently be the lower platen of a press, the two platens of which are adapted to be heated, for example by suitable electric heating elements 14 and 16 to a tempera ture of around 200 C. This temperature could be increased if desired to around 400, or a little'higherfbut since the melting point of tellurium is around 450 C. the temperature should be kept underthat.

At this high temperature, the upper press'pla'ten 15 is brought down on the powder and the pressure is brought 1 up to a high value, causing some sintering of the tellurium ency to deform the base plate at higher pressures, even when made-of steel. When softer base plate materials are used the pressure cannot be as high as in the case of steel base plates. It is desirable to have the opposed press platen surfaces of chrome-plated steel, as this provides a smooth surface which does not contaminate the powder. A sheet of soft metal such as aluminum could be placed under the press platen particularly if the press platens are not chrome plated, as this will tend to even out any irregularities in the tellurium layer 12. It may sometimes also be advantageous to place such an aluminum sheet between the base plate and the lower platen 13 for protection of the base plate against the high pressure. The time of the pressing can be in the order of about a minute, although the pressure could be maintained longer if desired. It may be possible to press at low temperatures than 200 C., for example, around 100 C. or even around room temperature; although in such cases the results are not as good as when using the higher temperatures, and the required time of pressing may be longer.

This heat-pressure operation will leave the tellurium as V a thin solid layer closely adherent to the base plate or its coating 11, the thickness of the tellurium being in the order of a few thousandths inch, such as .005 inch.

Following this application of the tellurium layer, a counter electrode in the form of a sheet or foil 18 of magnesium, or an alloy of magnesium with other hardening material, such as 2 or 3% of aluminum, and if desired some manganese, zinc or cadmium, is placed over the tellurium as shown in Fig. 5. The magnesium or magnesium alloy sheet may be around .025 inch thick, but greater thickness could be used if desired. It is preferably anodized. One way of doing this is to pickle the sheet in a chromic acid solution and then anodizing at a current of about 15 amperes per square foot for five or ten minutes in a solution of about NaOH and 0.3% phenol, by weight, in water held near the boiling temperature, using a stainless steel cathode. A suitable type is the grade available commercially as Dow FS or M, described in the booklet Magnesium Alloys and Products, published 1950 by Dow Chemical Co., Magnesium Division, Midland, Michigan. The magnesium oxide layer of sheet 18, represented by numeral 18a in Fig. 5, is placed adjacent to the tellurium surface. The upper or outer surface of the sheet 18, however, should have any oxide on it removed in order to allow good electrical contact since this constitutes one of the terminals of the rectifier.

It should be understood that an oxidized or anodized aluminum sheet could be used instead of the magnesium or magnesium alloy sheet 13. Cadmium or zinc could be substituted for the oxidized magnesium or aluminum, for low voltage elements.

It is not entirely clear what causes rectification in the tellurium element. I hold to the theory that there is probably little or no rectification in the tellurium itself, and that whatever rectification occurs is due to the nature of the boundary or barrier layer at the interface between the tellurium and the magnesium eounterelectrode. In particular, I believe it is due to the formation of a magnesium telluride at the boundary. In any event, the presence of the magnesium oxide at the boundary is important in the production of an effective barrier layer.

The silver coating against the tellurium surface at the base plate is also especially effective in producing good rectifying characteristics. While the silver may be dispensed with its presence materially enhances the rectification.

To establish the rectifying action, the device of Fig. 5 may be placed in a clamp pressing the counterelectrode toward the base plate with sufficient pressure to press the magnesium sheet or foil tightly against the tellurium surface, before application of a voltage to the element. This will tend to form the magnesium sheet to the element. The element may then be transferred to the position it will occupy during rectification. For example, if it be the type having a center hole (not shown) it may have a bolt passing through the hole with a nut or nuts at both sides of the rectifier to hold the element together, care being taken to provide insulation around the bolt to insulate it from plates 10 and 18 if the bolt be metallic. Or on the other hand if the clamp be non-conducting or insulated from one or both of plates 10 and 18 the element may be left in the clamp during the application of voltage.

Voltage may be applied by simply connecting an A. C. source across the rectifier preferably in series with some resistance. For example 6 volts A. C. in series with the rectifier element and about a 15 ohm resistance will serve the purpose. As soon as unidirectional current is observed, the rectifier is ready for use. Prolonged electroforming as in the case of selenium rectifiers is not found necessary in the tellurium rectifier. The flow of current will tend to fuse the counterelectrode 18 to the tellurium, thus tending to hold the components of the rectifier together even without a clamp.

By my invention I have succeeded in making commercially operable area type rectifiers by the use of tellurium as the semi-conductor and have thereby overcome the unsatisfactory performance resulting from previous attempts to use tellurium. For example, in attempting to melt or cast tellurium on or between plates as in the Hollnagel et a1. Patent 1,866,351 it was found that so much oxide and dross accompanied the melting and solidifying of the tellurium that its use was not practical. The dross would prevent close adherence of the tellurium to the base plate so that the tellurium would peel or chip off. Such rectifying effects as were obtainable by it were so erratic, as to prohibit its use, doubtless owing to the poor contact between the tellurium and the base plate. The formation of so much oxide and dross created so much impurity in the tellurium that its performance was highly unpredictable.

In my practice I have overcome the oxide and dross formation by avoiding any melting or casting of the tellurium. The heat-pressure treatment does not form the oxide, and I have found that it sinters or compacts the fine powder causing the particles to coalesce into the thin tellurium layer which is so closely adherent to the base plate as to effect good electrical contact, and providing consistently good rectifiers.

In the manufacture of selenium type rectifiers, selenium powder has heretofore been pressed at high temperatures to apply it to the base plate, but the pressing of selenium powder is quite different from that of tellurium powder. Selenium powder is amorphous and it flows at relatively low temperatures and pressures, causing the particles to How into each other. Tellurium, on the other hand, is metallic .and does not have the amorphous phase of selenium. Tellurium powder is simply a powder of the metal, for which reason it had heretofore been assumed that a heat-pressure treatment of it against the base plate would not produce an adherent layer with good rectifying properties. By my invention, however, I have discovered that the use of exceptionally high temperatures and pressures does enable tellurium powder to be formed into an adherent layer particularly when the grain size is small.

It should be understood that the invention is not limited to the particular structure and processes described and illustrated, which are merely by way of illustration rather than of limitation and that variations within the scope of the invention are permissible. The invention is not limited except in accordance with the scope of the appended claims.

I claim:

I. A dry plate rectifier comprising a metallic base plate, a layer in the order of a few thousandths inch thick of pressed sintered tellurium powder closely adherent to of magnesium oxide.

3. A dry plate rectifier comprising a ferrous metal I plate coated with silver, a layer of pressed tellurium powder closely adherent to the coated base plate, and a counter electrode of magnesium placed over the tellurium surface, said magnesium having a coating of magnesium oxide against the tellurium surface.

4. A dry plate rectifier comprising ametal base plate, a layer in the order of a few thousandths inch thick of coalesced compacted tellurium powder adherent to the base plate and an oxide coated magnesium sheet adherent overthe tellurium with the oxide against the tellurium.

5. A dry plate rectifier comprising a metallic base plate, a layer in the order of about .OOS-inch thick of compacted coalesced sintered tellurium powder adherent to the base plate and a metallic counterelectrode from the group consisting of aluminum, magnesium, cadmium and zinc fused to the tellurium surface, said counterelectrode having an oxide coating on its surface fused to the tellurium.

6. The method of making a rectifier which comprises compressing a layer of tellurium powder at a pressure at least as high as about 10,000 pounds per square inch and a temperature at least as high as about 200 C. to coalesce the powder and cause it to adhere to the base plate as a hard coherent layer, pressing an oxide coated magnesium foil over the tellurium surface with the oxide against the tellurium and then applying a voltage be tween the baseplate and the foil until a unidirectional current flows, thereby fusing the foil to the tellurium.

7. The method of making a rectifier which comprises compressing a layer of tellurium powder to a base plate at a pressure at least as high as about 10,000 pounds per square inch to coalesce the powder and cause it to adhere to the base plate as a hard coherent layer, pressing a metallic counterelectrode sheet over the tellurium surface and then applying a voltage between the base plate and the counterelectrode until a unidirectional current flows thereby fusing the counterelectrode to the tellurium.

8. A dry plate rectifier comprising a metallic base plate, a layer in the order of about .OOS-inch thick of compacted coalesced sintered tellurium powder adherent to the base plate and a metallic counterelectrode adherent to the side of the tellurium layer opposite the base plate.

References Cited in the file of this patent UNITED STATES PATENTS 1,866,351 Hollnagel'et al July 5, 1932 1,961,825 Harty June 5, 1934 1,994,632 Becker Mar. 19. 1935 2,137,316 Geel et a1. Nov. 22, 1938 2,307,474 Thompson Jan. 5, 1943 2,375,181 1 Williams May 1, 1945 

1. A DRY PLATE RECTIFIER COMPRISING A METALLIC BASE PLATE, A LAYER IN THE ORDER OF A FEW THOUSANDTHS INCH THICK OF PRESSED SINTERED TELLURIUM POWDER CLOSELY ADHERENT TO THE BASE PLATE AND A MAGNESIUM COUNTERELECTRODE OVER THE TELLURIUM SURFACE, SAID COUNTER ELECTRODE HAVING A LAYER OF MAGNESIUM OXIDE ADJACENT TO THE TELLURIUM.
 6. THE METHOD OF MAKING A RECTIFIER WHICH COMPRISES COMPRESSING A LAYER OF TELLURIUM POWDER AT A PRESSURE AT LEAST AS HIGH AS ABOUT 10.000 POUNDS PER SQUARE INCH AND A TEMPERATURE AT LEAST AS HIGH AS ABOUT 200* C. TO COALESCE THE POWDER AND CAUSE IT TO ADHERE TO THE BASE PLATE AS A HARD COHERENT LAYER, PRESSING AN OXIDE COATED MAGNESIUM FOIL OVER THE TELLURIUM SURFACE WITH THE OXIDE AGAINST THE TELLURIUM AND THEN APPLYING A VOLTAGE BETWEEN THE BASE PLATE AND THE FOIL UNTIL A UNIDIRECTIONAL CURRENT FLOWS, THEREBY FUSING THE FOIL TO THE TELLURIUM. 